Group III nitrides are composed of nitrogen (N) in combination with one or more of the periodic table Group III elements: aluminum (Al), gallium (Ga), and indium (In). There is substantial interest in Group III-nitride materials and devices due to applications in optoelectronics, portable consumer devices such as handheld projectors, high resolution televisions, displays, and lighting. Specifically, Group III-nitride laser diodes are demanded for many portable consumer devices such as handheld projectors, high resolution televisions, displays, and lighting.
Major challenges, however, exist in manufacturing GaN-based laser diodes such as (1) material defects (−109/cm2), (2) intrinsic polarization fields (−MV/cm), and (3) non-ideal cavity mirror formation due to lack of cleavage planes. As GaN epitaxy is grown on highly lattice-mismatched substrates such as sapphire, silicon, or silicon carbide (SiC), there is a twist and asymmetry between the substrate and the GaN regrowth leading to a high incidence of material defects. Accordingly, processing employs either dry etching or polishing to form the mirrors due to the high incidence of material defects.
Polarization is the inherent non-centrosymmetric characteristic of GaN crystal. Along the common growth direction of <0001>, wurtzite (e.g., hexagonal) phase GaN material is inherently polar, which degrades the recombination characteristics in lasers because of the lack of efficiency of recombination across non-aligned electron and hole wavefunctions. Nonpolar GaN freestanding substrates are available, which may address the above issues, but high costs make this option unworkable. A disadvantage with laser diodes using non-polar surfaces is the difficulty in obtaining nonpolar GaN substrates (such as m-plane discussed below). Cutting a non-polar GaN substrate requires consuming a two-inch piece of polar GaN, with a value of about $10,000, and the area of the cut GaN substrate is quite small. Currently, the area is about 3 mm to 5 mm, and no larger than about 10 mm. What is desired, therefore, is a mass-producible, cost-effective approach that eliminates piezoelectricity effects in GaN and other Group III-nitrides.